* Aramco wants anchor investors to cover no less than 40% -source. : DUBAI/RIYADH (Reuters) - Oil giant Saudi Aramco’s much-vaunted stock market listing was delayed after deal advisers said they need more time to lock in cornerstone investors, three sources with direct knowledge of the matter told Reuters. After a false start last year, preparations gathered momentum this summer with approaches to sovereign wealth funds, rich Saudis and large foreign fund managers as potential cornerstone investors only for plans to unravel for a second time.